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The stupefying pace of glacier melt in the 1940s

The most recent studies by researchers at ETH Zurich show that in the 1940s Swiss glaciers were melting at an even-faster pace than at present. This is despite the fact that the temperatures in the 20th century were lower than in this century. Researchers see the main reason for this as the lower level of aerosol pollution in the atmosphere.

Simone Ulmer

A glaciologist on the way to work on the Silvretta glacier (Image: Matthias Huss / ETH Zurich)
(large view)

In Switzerland,
the increase in snow in wintertime and the glacier melt in summertime have been
measured at measurement points at around 3,000 metres above sea level – on the
Clariden Firn, the Great Aletsch glacier and the Silvretta glacier – without
interruption for almost 100 years. As part of his doctoral work, Matthias Huss
used this unique range of measurements to examine how climate change in the
last century affected the glaciers. The work was carried out under the
supervision of Martin Funk, professor and head of the Department for Glaciology
at the Laboratory for Hydraulics, Hydrology and Glaciology (‘VAW’) at ETH Zurich, who is also
co-author of the study.

Solar radiation as the decisive
factor

In its work, the research team took into account the solar radiation
measured on the Earth’s surface in Davos since 1934. Studies over the past two
decades have shown that solar radiation varies substantially due to aerosols
and clouds, and this is assumed to influence climate fluctuations. Recent years
have seen the emergence of the terms ‘global dimming’ and ‘global brightening’
to describe these phenomena of reduced and increased solar radiation
respectively. These two effects are currently the subject of more and more
scientific research, in particular by ETH Zurich, as experts feel that they
should be taken into account in the climate models (see ETH Life dated July 9, 2009)

The new study, published in the journal ‘Geophysical Research Letters’,
confirms this requirement. This is because, taking into account the data
recorded for the level of solar radiation, the scientists made a surprising
discovery: in the 1940s and in the summer of 1947 especially, the glaciers lost
the most ice since measurements commenced in 1914. This is in spite of the fact
that temperatures were lower than in the past two decades. “The surprising
thing is that this paradox can be explained relatively easily with radiation”,
says Huss, who was recently appointed to the post of senior lecturer at the
Department of Geosciences at the University
of Fribourg in Switzerland.

On the basis of their calculations, the researchers have concluded that
the high level of short-wave radiation in the summer months is responsible for
the fast pace of glacier melt. In the 1940s, the level was 8% higher than the
long-term average and 18 Watts per square metres above the levels of the past
ten years. Calculated over the entire decade of the 1940s, this resulted in 4% more
snow and ice melt compared with the past ten years.

Furthermore, the below-average melt rates at the measurement points
during periods in which the glacier snouts were even advancing correlate with a
phase of global dimming, between the 1950s and the 1980s.

Less snow fall and longer melt
periods

The researchers arrived at their findings by calculating the daily melt
rates with the aid of climate data and a temperature index model, based on the
half-yearly measurements on the glaciers since 1914. These results were then
compared with the long-term measurements of solar radiation in Davos.

Huss points out that the strong glacier melt in the 1940s puts into
question the assumption that the rate of glacier decline in recent years “has
never been seen before”. “Nevertheless”, says the glaciologist, “this should
not lead people to conclude that the current period of global warming is not
really as big of a problem for the glaciers as previously assumed”. This is because
it is not only the pace at which the Alpine glaciers are currently melting that
is unusual, but the fact that this sharp decline has been unabated for 25 years
now. Another aspect to consider – and this is evidenced by the researchers’
findings – is that temperature-based opposing mechanisms came into play around
30 years ago. These have led to a 12% decrease in the amount of precipitation
that falls as snow as a percentage of total precipitation, accompanied by an
increase of around one month in the length of the melt period ever since this
time. Scientists warn that these effects could soon be matched by the lower
level of solar radiation we have today compared with the 1940s.

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